This invention relates in general to clock and data recovery systems, and more particularly, to digital data separators.
To receive a transmitted digital data stream, it is necessary to recover the clock signal of the data stream in order to synchronize to the stream. Once synchronization to the stream is established, the relative locations of the individual pulses of that stream are known and the data contained in the stream can be recovered.
In cases where the data stream exhibits jitter and speed variations, such as when the stream originates from a floppy disk or magnetic tape, the recovery of data can be impaired by these conditions. For example, when a data stream is encoded in the known MFM (Modified Frequency Modulated) format and the stream originates from a floppy disk or magnetic tape, the pulses which form the digital data stream are made more difficult to receive by these undesirable conditions. In this encoding technique, the data pulses are derived from the sensing of magnetic flux changes recorded on magnetic media. These data pulses are susceptible to a "peak shift jitter" phenomena and other forms of data jitter. Moreover, the digital data stream is susceptible to both long and short term speed variations resulting from motor speed variation in the disk or tape drive, movement of an elastic medium over a surface with friction and other sources. For these reasons, in order for a digital data separator to operate effectively, the digital data separator should tolerate a large degree of jitter in the data stream. The digital data separator should also have the ability to track long and short term speed variations.
In the design of digital data separators, jitter tolerance and speed tracking are opposed goals that are traded off after a certain point. It is desirable to have a digital data separator which uses as much information as possible to meet these opposing goals. Loss of information through round-off error, quantization error such as determining when the first input data pulse has arrived, or other sources can directly result in the degradation of performance of a digital data separator.